Device having a point and a spatial sound generating-means for providing stereo sound sensation over a large area

ABSTRACT

A device is described, having a first ( 2 ) and a second ( 4 ) sound-generating means and an input for a stereo signal (S) comprising left (L) and right (R) sound signals. The device has an interconnected first ( 1 ) and second ( 3 ) part comprising a first ( 2 ) and a second ( 4 ) sound-generating means, respectively. The first part ( 1 ) is formed so as to couple soundwaves generated by the first sound-generating means ( 2 ) into a surface ( 6 ) when placed upon said surface ( 6 ), and the device has means ( 5 ) for sending a first signal (S 1 ), which is a composite of the left (L) and right (R) sound signals, to the first sound-generating means ( 2 ) of the first part ( 1 ), and a second signal (S 2 ), which is a different composite of the left (L) and right (R) sound signals, to the second sound generating means ( 4 ) of the second part ( 3 ).

FIELD OF THE INVENTION

The invention relates to a device having a first and a secondsound-generating means and an input for a stereo signal comprising leftand right sound signals.

BACKGROUND OF THE INVENTION

Devices comprising two sound-generating means are known and widely used.

Spatial localization of sound has always been considered to be ofparamount importance in audio reproduction devices. Contemporary soundreproduction devices are required to have at least stereo playbackcapabilities, and two or more loudspeakers are conventionally providedfor such facilities. This usually results in devices with externalloudspeakers separated from each other by a considerable distance andthe associated footprint and wiring issues. However, this requires aspace with enough distance between the loudspeakers, which is not alwaysavailable and often also requires wiring. In many circumstances, morecompact devices would be preferred. However, although such compactdevices have been developed and sold, the stereo playback of thesedevices such as ‘ghettoblasters’ is not perceived as true stereoplayback because the loudspeakers are spaced very close to each other.Some stereo-widening techniques such as Philips' “Incredible Surround”are known and overcome such limitations to some extent. However, suchtechniques may typically reduce the “sweet spot” (i.e. the area in whicha good or acceptable stereo sound is produced), which sometimes becomesimpractically small since consumers typically do not sit exactly infront of such a sound device. Furthermore, such techniques are usuallycomplicated. Typically, compact devices for producing stereo sound havethe drawback that, although they do provide stereo sound only in or nearthe sweet spot, the positions in which this is achieved is limited tousually a rather small area right in front of the device. For truestereo sound perception, it is highly preferred that the stereo soundperception remains when a listener walks around or, when more than oneperson is listening to the sound, all listeners are provided withsubstantially the same quality of sound. The stereo signal has a leftand a right sound signal. The denotations “left” and “right” areunderstood to merely indicate one of the usual sub-signals of a stereosignal (wherein “stereo signal” may be any multi-channel signal). Unlessspecifically described otherwise, these denotations are not understoodto be anything else but such a simple division of the stereo signalusing common terms, and not an undue restriction. In a simpleembodiment, however, “left” and “right” stand for the usual “left” and“right” stereo channels.

Thus there is a need for a device that is capable of giving a stereosound sensation to the listener over a relatively large area while yetbeing a, preferably very, compact sound reproduction device.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a compact sound reproductiondevice capable of giving a stereo sound sensation over a relativelylarge area.

To this end, the device in accordance with a first aspect of theinvention is characterized in that it has an interconnected first andsecond part comprising a first and a second sound-generating means,respectively, the first part being formed so as to couple soundwavesgenerated by the first sound-generating means into a surface when placedupon said surface, and wherein the device has means for sending asignal, which is a composite of the left and right sound signals, to thefirst sound-generating means of the first part, and a signal, which is adifferent composite of the left and right sound signals, to the secondsound-generating means of the second part.

The invention is based on the following recognition.

Stereo music typically has a left and a right channel (L, R). Theinventors have realized that by forming the first part in such a waythat the soundwaves are effectively coupled into a surface on which thefirst part may be placed, e.g. a table, said object on which the firstpart is placed will be excited and vibrate with the sound generated bythe first part. Due to the large size of the vibrating object, the firstsignal, which is a composite of the left and right sound signal, isperceived as a “volume sound”. Another signal, also a composite of theleft and right sound signal, but a different one, is sent to the secondpart, which basically acts as a point source. Wherever the listener isseated around the table, the sound from both sources, however, soundsthe same. The result is that the sound perception is the same all aroundthe device. There is not one sweet spot. The inventors have found that asurprisingly good stereo perception can be obtained. The signals sent tothe first and the second part are different, since the inventors havefound that certain sounds attribute more to the volume sound effect thanothers.

More in particular, for preferred embodiments, the first signal sent tothe first part and the second signal sent to the second part are, inoperation, substantially orthogonal signals, i.e. when the first signalis expressed as S1=aL+bR, and the second signal as S2=cL+dR, where L andR are the left and right sound signal, respectively, the product (ac+bd)is on average substantially zero, at least less than 0.1, preferablyless than 0.05, wherein most preferably the absolute values of a and care approximately (within twenty to ten percent) the same, as are theabsolute values of b and d.

In a preferred embodiment, the first signal is mainly comprised of adifference of the left and right stereo sound signals (L−R) and thesecond signal sent to the second part, and the point source is mainlycomprised of a sum signal of the left and right stereo sound (L+R). Inanother preferred embodiment, the signals may be analyzed to find adominant signal (aL+bR), and the device has means for sending a dominantsignal and the residual signal. Most music comprises signals that arepresent in both stereo signals, typically, for instance, a singer or asolo artist. The sound produced by the solo artist is usually thedominant signal, and usually the solo artist stands in the middle, i.e.the intensities for the sound produced by the solo artist are the samein the left and the right sound signal. Summing the stereo signals (L+R)and sending the sum (L+R) to the second sound-generating means will leadto the solo artist being heard as if he were present at the position ofthe second part, i.e. a localized source for the singer or solo artistis established. The sound produced by this localized source is perceivedsubstantially the same around the source. The sounds that give a stereoimpression to the music are typically present at either the left or theright signal or at least much more in one channel than in the other.Sending the difference signal (L−R) to the first sound-generating meanswould in itself not necessarily give a sufficient stereo soundimpression, since both the first part (L−R source) and the second part(L+R source) would generate sound at a single point, where usually thesepoints are close to one another for a compact device. This would lead tothe same problems as described above for existing devices. In the deviceaccording to the invention, it is possible, in operation, to effectivelycouple this sound via the first part into a surface on which the firstpart may be placed, e.g. a table, whereby said object on which the firstpart is placed will be excited and vibrate with the sound generated bythe first part. The result is that the table or other objectco-vibrating with the second part forms a spatially extended sourcegenerating the difference signal (L−R). Wherever the listener is seatedaround the table, the sound from both sources sounds the same. Thecombination of a localized source for one composite signal, preferablythe dominant signal, for instance, the sum signal (the second part L+R)and a spatial source for another composite signal, preferably theresidual signal, for instance, for the difference signal (L−R) (thefirst part in combination with a vibrating, excited, surface) produces astereo sound impression all around. The electronics for the presentdevice are very simple, the device itself may thus be very compact. Yeta stereo sound impression is achieved which does not require thelistener to be positioned in a particular spot or area. Use of a sum anda difference signal is a preferred embodiment.

The invention makes use of the possibility to vibrate a large rigidobject, for example a table top, by means of a much smaller primarysource, such that it produces a larger sound than the sound originatingfrom the primary source if sufficient acoustical coupling is ensured.Depending on the properties of the object to be excited and theacoustical coupling, the sound intensity of the larger object is largerand richer than that of the primary source (the first sound-generatingmeans) alone, presumably because of the much larger surface area of thetable when compared to the primary source. In this application, thisphenomenon is also called co-vibration or co-excitation.

Within the concept of the invention, the device is made to be such thata table vibrates when the first part is positioned on the table oranother co-vibrating object. Such effects always occur to some extent.However, normally, each loudspeaker box is made to minimize such aneffect as much as possible, in other words, co-vibration is counteractedor minimized as much as possible. In conventional designs, theloudspeakers do not or hardly make contact with the table. They areusually suspended in an enclosure which, in fact, comprises a goodvibration damper or functions as such. However, in contrast, theco-vibrating effect is an integral part of the present invention.Measures are described hereinafter to distinguish devices according tothe invention from those beyond its scope.

The first part of the device and the first sound-generating means arearranged in such a way that, when the first part is positioned on atable top as defined in this application, the sound volume produced bysaid first part at a distance of 1 meter from said first part isincreased by at least 6 dB as compared to the same part when used inair. Simply picking up the first part will therefore distinguish thosedevices within and beyond the scope of the present invention. The deviceis laid on a wooden (plywood) table top having a thickness of 18 mm anda size of 90*180 cm, more or less corresponding to a standard officetable, and a force corresponding to a weight of 100 grams is applied tothe device while laying flat on the table, with the display parallel tothe table, and the increase of sound intensity is measured at a distanceof 1 meter with respect to the same device when in air, i.e. lying onwool. The 100 grams include the force applied by the device itself Inmany circumstances, this will be approximately the weight of the firstpart itself.

The manner in which the increase of sound intensity is measured isdefined in this application.

The first part preferably comprises a coupling means. Using a couplingmeans, i.e. a means for enhancing mechanical coupling, very substantialincreases of sound intensity of the first sound-generating means, above15 dB, or even above 20 dB are possible. Such means may e.g. be suctionmeans or magnets. Suction means will effectively increase the force bymeans with which the device sticks to the table (enhancing its apparentweight), while magnets will enhance its apparent weight when put on asteel surface. Both may be present.

The mechanical coupler is preferably formed in such a way that itextends slightly beyond the first means proper so that, when the firstmeans is positioned on the table or other flat surface, the first meansrests on the mechanical coupler. The mechanical coupler itself does nothave much influence on the sound intensity of the device when hand-held.

Due to excitation of the object on which the first part is placed, amuch richer and better audible sound is obtained.

The standard test with which the increase can be measured will beexplained in the description of the Figures. Basically, the increase ofsound volume is measured at a distance of 1 meter, while the device isplaced on a wooden table of 18 mm thickness and 90*180 cm size, whilethe total weight of the device plus additional pressure exceeds 100grams.

In preferred embodiments, the device is provided with a sound-recordingelement and the device comprises a means for establishing a comparisonbetween a registered sound and an emitted sound signal and means toindicate that the device is in co-excitation with another object and toregulate the sound intensity of the first means accordingly. It is notknown in advance on what surface the first means will be positioned.Thus, the effective sound intensity may vary, depending on the table onwhich the first means is positioned. By providing a microphone, and afeedback of the actual sound intensity, some means for regulating thesound intensity in effect produced by the first means is possible.

In preferred embodiments, the second sound-generating means ispositioned on a swivel, i.e. a means for changing the direction of thesound produced by the second sound-generating means. Such a swivel(which, within the scope of the invention, comprises any means forchanging the position or direction of the second sound-generating meansvis-à-vis the first, while yet maintaining a physical connection) maye.g. be used advantageously to direct the sound in one generaldirection.

In another related aspect of the invention, the device comprises aninterconnected first and second part comprising a first and a secondsound-generating means, respectively, the first part being formed so asto couple soundwaves generated by the first sound-generating means intoan outer envelope of the first part, and wherein the device has meansfor sending a first signal, which is a composite of the left and rightsound signals, to the first sound-generating means of the first part,and a second signal, which is a different composite of the left andright sound signals, to the second sound-generating means.

In a further related aspect of the invention, the device comprises aninterconnected first and second part comprising a first and a secondsound-generating means, respectively, the first part being formed so asto couple soundwaves generated by the first sound-generating means intoan elongated element coupled to the first part, and wherein the devicehas means for sending a signal, which is a composite of the left andright sound signals, to the first sound-generating means of the firstpart, and a signal, which is a different composite of the left and rightsound signals, to the second sound-generating means.

The inventors have realized that a similar advantage may be obtained byusing either an outer envelope of the first part as a co-vibratingobject, or an elongated element (i.e. an object having a dimension whichis larger than the dimension of the first part itself).

These and other objects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates schematically a device according to the invention

FIG. 2 illustrates the basic concept of the invention.

FIGS. 3A and 3B illustrate schematically further embodiments of a deviceaccording to the invention.

FIG. 4 illustrates schematically yet a further embodiment of a deviceaccording to the invention.

FIG. 5 illustrates a device according to the invention, including asound co-vibration element.

FIG. 6 illustrates the experimental set-up for measuring soundenhancement by acoustical coupling.

FIG. 7 illustrates a further embodiment of a device according to theinvention.

FIG. 8 illustrates a further embodiment according to the invention.

The Figures are not drawn to scale. Generally, identical components aredenoted by the same reference numerals in the Figures.

DESCRIPTION OF EMBODIMENTS

FIG. 1 schematically shows a compact stereo device according to theinvention.

The stereo device has an input for an incoming stereo signal Scomprising a left (L) and a right (R) signal, and an interconnectedfirst (1) and second part (3) comprising a first (2) and a second (4)sound-generating means, respectively. The first part 1 is formed so asto couple soundwaves generated by the first sound-generating means intoa surface when placed upon this surface. Basically, in this example,soundwaves are effectively coupled with the housing and via the housingwith, for instance, a table top or directly into the table top.Normally, sound-generating means are positioned inside loudspeakers sothat they are decoupled with the housing and the outside world as muchas possible. In the first part, the opposite effect is sought, a largecoupling is present to the outer envelope, to an elongated element or toa surface upon which the first part is placed. The device has means 5for sending a composite signal S2 (L+R), comprising in this example asum signal of the first (L) and second (R) stereo signals, to the secondsound-generating means (4) of the second part (2), and another,different composite signal S1 (L−R), comprising in this example adifference signal of incoming first (L) and second (R) stereo signals,to the first sound-generating means (2) of the first part (1). Thecomposite signals are preferably orthogonal signals, i.e. when the firstsignal S1=aL+bR and the second signal S2=cL+dR, then ac+bd≈0. Using asum and a difference signal is a simple embodiment. The device may havemeans for establishing a dominant signal, and send it to the second part(the “point-source”), and send the residual signal (a signal orthogonalto the dominant signal) to the first means (the “spatial source”). Insimple designs, the different composites S1, S2 of the signals may bethe same throughout the frequency range. In more complex embodiments,the values a, b, c and may differ for different frequency ranges. In anembodiment, for instance, the first and the second part may receive thesame signal for a lower frequency range up to the cut-off frequency,e.g. up to 300 or 500 Hz, and orthogonal signals at all frequenciesabove the cut-off frequency.

The first sound-generating means (2) of the first part (1) may be, andin preferred embodiments is, a piezo-element, which is so positionedthat, when the device is placed on a surface, the piezo-element rests onthis surface. In this manner, vibrations are effectively coupled intothe surface. A piezo-element is a preferred element for the first partbecause piezo-elements have such a form and function that they are wellsuited to couple soundwaves into a surface. The second sound-generatingmeans (4) of the second part (2) may be a squeeter. The means 5 may beattached to the first and the second part forming a unit. In suchembodiments, the unit receives the signal S and the sum and thedifference signal are generated in situ, i.e. in the unit. In otherembodiments, the means 5 is separate from the first and the second part.For instance, there may be one central processing unit (such as aCD-player) and several sound units, wherein a means 5 is provided at thecentral processing unit for providing the signals, which are thenwirelessly sent to the units.

FIG. 2 illustrates the basic concept of the invention in the firstaspect. The invention is based on the recognition that it is possible tovibrate a large rigid object, for example, a table top 6, by means of amuch smaller primary source (first sound-generating means 2), such thatit produces a larger sound than the sound originating from the primarysource if sufficient acoustical coupling is ensured. Depending on theproperties of the object to be excited and the acoustical coupling, thesound intensity of the larger object is larger and richer than that ofthe primary source (the first sound-generating means) alone, presumablybecause of the much larger surface area of the table when compared tothe primary source. Thus, even a lower frequency sound can be produced,even though the excitation amplitude of the table is much smaller (a fewnm) than that of the primary source itself (several microns to tens ofmicrons). In order for this to be achieved, a positive, sufficientlylarge acoustical coupling between the sound-generating means and thelarger object needs to be established. In this application, thisphenomenon is also indicated by the words co-vibration and/orco-excitation. FIG. 2 illustrates schematically that the secondsound-generating means produce a sound which originates more or lessfrom a single point, whereas due to the co-vibration of the table top,illustrated by the vertical arrows, an extended sound source,illustrated by the arrows emanating from the element 6, is created.

Sending the sum (L+R), or in another preferred embodiment, the dominantsignal, to the second sound-generating means (4) will lead to the soloartist being heard as if he were present at the position of the secondpart, i.e. a localized source for the singer or solo artist. The soundproduced by this localized source is perceived as being substantiallythe same all around the source. The sounds that give a stereo impressionto the music are typically present at either the left or the rightsignal, or at least much more in one channel than in the other, or inthe residual signal, or in anti-phase, or decorrelated. Sending thedifference signal (L−R), or the residual signal, to the firstsound-generating means (2) would in itself not necessarily give asufficient stereo sound impression, since both the first part(L+R-source) and the second part (L−R source) would generate sound at asingle point, where, for a compact device, these points are usuallyclose to one another. In the device according to the invention, thesoundwaves, when positioned on a surface, are effectively coupled into asurface on which the first part is placed, e.g. a table. The object onwhich the first part is placed will vibrate with the sound generated bythe first part. The result is that the table or another object itselfforms a spatially extended source, as illustrated in FIG. 2 reproducingthe difference signal (L−R). Wherever the listener is seated around thetable or object in general, the sound from both sources sounds the same.The combination of a localized source for the sum signal (the secondpart) and a spatial source for the difference signal (the first part incombination with a vibrating surface) produces a stereo soundimpression. This stereo sound impression is substantially the samearound the co-vibrating object. The electronics are very simple, as isthe device, which itself may thus be very compact. A stereo soundimpression is achieved which does not require the listener to bepositioned in a particular spot. It is to be noted that where mention ismade of the “sum” and the “difference” signal, this is meant to expressthat the signal sent is mainly comprised of the sum and/or differencesignal.

FIGS. 3A and 3B illustrate a further embodiment of a device according tothe invention. The device comprises a coupling means 7, 8 for couplingthe first part of the device to a surface. Such a coupler may be, forinstance, a suction device 7. Some suction force will increase thecoupling between the device and the surface. The coupling means may alsobe in the form of magnets 8. Provision of such magnets will increase thecoupling upon a steel surface. The coupling means may combine bothfunctions, for instance, when suction devices with magnets enclosed areused.

FIG. 4 illustrates a further embodiment. In this embodiment, the firstand the second part are interconnected in such manner that the firstpart may be moved and oriented with respect to the second part. Somedirectionality in the second sound source may thereby be imparted.

FIG. 5 illustrates another aspect of the invention in which the deviceitself comprises a co-vibrating element 51. In embodiments, such as inthe previous Figures, the unit of first and second sound-generatingmeans will be placed on a table to produce stereo sound. In theembodiment of FIG. 5, the device itself comprises the vibrating means.An example of such a means is a table 51 which comprises a built-infirst and second part. This may be in the form of a unit comprising afirst and a second part, and an extended element, wherein the extendedelement and the unit comprise fastening means to mechanically fasten theunit and the extended element. In a preferred embodiment, the fasteningmeans are reversible, i.e. the unit may be decoupled from the extendedelement. This would allow decoupling of the unit and placing it onanother element or e.g. a table. A further example of a device similarto the one shown in FIG. 5 would be an overhead set, wherein theextended element would be part of, or parallel to, the ceiling. Thesound would then come from above. The advantages of a device that has aco-vibrating element built in (be it the outer envelope of the firstpart or an elongated element to which the first part is coupled) is thatthe coupling is known.

FIG. 6 schematically indicates the manner in which the sound increase ismeasurable. The device 1 is positioned on a table 51 in the properorientation and, if it has a coupling means 7, 8, with the couplingmeans on the table, and if the device has a suction cup, with suctionaction.

The sound level at 1 m distance is measured at frequencies of 1 and 2kHz, while the first part receives a signal at mid-range of thedynamical range of the first part. The device is removed from the tableand placed on a woolen cloth or suspended in air. The sound level isagain measured, using, of course, the same signal and the same distanceand orientation of the sound-recording means and device. If the increaseof the sound level for said frequencies is more than 6 dB, the devicefalls within the scope of the claim. If not, it falls outside the claim.The increase is preferably at least 15 dB, more preferably at least 20dB. The table has a wooden (plywood) table top of 90*180 cm with athickness of 18 mm. This corresponds more or less to a standard tabletop in offices.

In the standard test, a table having a plywood table top of 18 mm and asize of 90*180 cm is used. However, this is merely for the purpose ofestablishing a bench mark. Experiments have shown that, when this is thecase, very similar results are obtained e.g. when a table of 14 mmplywood and size 160*80 cm is used or when it has a steel table top anda size of 100*200 mm.

The standard of the plywood table having the specified measures is usedto establish a frame of reference, while the values for other types oftable tops are roughly similar.

It is to be noted that, within the concept of the invention, the deviceis made in such a way that a table vibrates when the first part ispositioned on a table or other co-vibrating object. Such effects alwaysoccur to some extent. However, normally, each loudspeaker box is made tominimize such effects as much as possible, in other words, co-vibrationis counteracted or minimized as much as possible. In conventionaldesigns, the loudspeakers do not or hardly make contact with the table.They are usually suspended in an enclosure which, in fact, comprises avery good vibration damper or functions as such. In contrast, the effectof co-vibration is an integral part in the present invention. Themeasurement described above, which can easily be performed by any personskilled in the art, clearly distinguishes devices according to theinvention from those outside its scope. For a device into which anelongated element is built (as in FIG. 5), the contribution to the soundof the vibrating element is easily measurable by measuring the soundproduced by the device when a signal is sent to the first part, and thenby measuring again while clamping down the element (so that it cannotvibrate, a heavy weight could, for instance, be put on the element) andsubsequently by dividing the two measurements by dividing theintensities. If it is more than 6 dB, preferably more than 15 dB, thedevice is a device according to the invention. When the outer envelopeof the first part forms the co-vibrating element, a similar test may beperformed, in which a signal is sent to the first part in normaloperation and mid-range, the sound intensities are measured,subsequently the outer envelope is clamped down so that it cannotvibrate, the sound intensities are measured again, and the measuredsound intensities are divided.

FIG. 7 illustrates a preferred embodiment of the invention. Theco-vibrating surface may change the frequency distribution of the sound,since certain frequencies may be more amplified than others. FIG. 7illustrates a device comprising a sound-recording means 71, whichrecords the sound generated. In comparator C, this sound is comparedwith the original sound (slightly delayed in time, to account for thetime difference). The measured difference in intensity and e.g.frequency distribution of intensity, is fed back to amplifier A tochange the signal to the sound-generating means, such that the endresult is that the recorded sound corresponds to the original signal, ofcourse within measuring accuracy. It is to be noted that in that casethe signal fed to first sound-generating means will be equivalent toL−R, but not exactly the same, thus illustrating an example of thecircumstances, as already mentioned above that, where mention is made ofa sum or difference of a signal being sent to the first (second)sound-generating means, such is meant to indicate the general content ofthe signal, but should not be interpreted so restrictively as to bepurely the sum or difference signal.

In the embodiments shown, the first part and the second part areinterconnected. In a preferred embodiment, this means that they arephysically interconnected, in which the two parts form an integral unit.However, this does not mean that the unit may not comprise means fordecoupling the first and the second part. In embodiments, this may beadvantageous, e.g. because this would make it possible to provide thesecond means above a table and the first means on a table. However, thetwo parts would still form a unit in the sense that they areinterconnectable to form one unit, and that the signals are coupled.

FIG. 8 illustrates an embodiment in accordance with another aspect ofthe invention, in which the first part comprises a firstsound-generating means which is coupled to an outer envelope (housing)81 of the first part. The outer envelope functions as a spatial source.

It will be evident that many variations are possible within theframework of the invention. It will be appreciated by persons skilled inthe art that the present invention is not limited by what has beenparticularly shown and described hereinbefore. The invention resides ineach and every novel characteristic feature and each and everycombination of characteristic features. Reference numerals in the claimsdo not limit their protective scope. Use of the verb “to comprise” andits conjugations does not exclude the presence of elements other thanthose stated in the claims. Use of the article “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.

It is to be noted with reference to the claims that variouscharacteristic features defined in the set of claims may occur incombination.

1. A device having a first and a second sound-generating means and aninput for a stereo signal comprising left and right sound signals,wherein the device has an interconnected first and second partcomprising the first and the second sound-generating means,respectively, the first part being physically connected to the secondpart, the first part being formed so as to couple soundwaves generatedby the first sound-generating means into a surface when said device isplaced upon said surface, wherein coupling soundwaves into the surfaceresults in a co-vibrating of (i) the first sound-generating means and(ii) the surface to form (iii) a spatially extended source, and whereinthe device has means for sending a first signal, being a first compositeof the left and right sound signals, to the first sound-generating meansof the first part, and a second signal, being a second composite of theleft and right sound signals different from said first composite, to thesecond sound-generating means of the second part, wherein responsive tothe co-vibrating of the first sound-generating means and the surface, asound volume produced by said first part and said surface at a distanceof one (i) meter from said first part is increased by at least 6 dB ascompared to a sound volume produced by the first part when used in airand not coupled to said surface, and wherein, due to the soundwaves fromthe spatially extended source of the first sound-generating means andthe surface, and the soundwaves from the second sound-generating means,the device provides a stereo sound sensation over a large area.
 2. Thedevice as claimed in claim 1, wherein the means for sending is arrangedin such a way that the first signal and the second signal aresubstantially orthogonal signals.
 3. The device as claimed in claim 2,wherein the means for sending is arranged in such a way that the firstsignal comprises a difference signal of the left and right stereosignals, and the second signal comprises a sum signal of the left andright stereo signals.
 4. The device as claimed in claim 1, wherein thefirst part comprises coupling means for coupling the first part to thesurface.
 5. The device as claimed in claim 4, wherein the coupling meanscomprises a suction element.
 6. The device as claimed in claim 4,wherein the coupling means comprises a magnet.
 7. The device as claimedin claim 1, wherein the first sound-generating means comprises apiezo-element.
 8. The device as claimed in claim 1, wherein the secondsound-generating means comprises a localized source for producing soundthat is perceived substantially the same around the localized source. 9.The device as claimed in claim 8, wherein the first and second signalscomprise residual and dominant signals, respectively, and wherein acombination of the localized source and the spatially extended sourceproduce a stereo impression all around.
 10. The device as claimed inclaim 1, further wherein the sound volume is increased by at least 15 dBwhen the first part is coupled to the surface.
 11. The device as claimedin claim 1, further wherein the sound volume is increased by at least 20dB when the first part is coupled to the surface.
 12. The device asclaimed in claim 1, wherein the second sound-generating means ispositioned on a swivel for changing a direction of the sound produced bythe second sound-generating means vis-à-vis the first sound-generatingmeans.
 13. The device as claimed in claim 1, wherein the surfacecomprises a surface of an elongated element.
 14. The device as claimedin claim 13, wherein the elongated element comprises an object having adimension which is larger than a corresponding dimension of the firstpart.
 15. The device as claimed in claim 13, wherein the first part andthe elongated element are coupled by reversible coupling means.
 16. Thedevice as claimed in claim 1, wherein the surface comprises an outerenvelope of the first part, and wherein the outer envelope is configuredto operate as a co-vibrating object.